The present invention relates generally to a technique in digital mobile communications and, more particularly, to a technique for canceling interference in a loudspeaker communication path through adaptive discrimination.
In a digital mobile phone, communications are conducted through two possible communication paths. In the first communication path, a microphone of the mobile phone picks up the voice activity of a human user, the subsequent voice activity is converted to an electrical signal, the electrical signal is converted by an analog-to-digital converter into a digitized information stream, the digitized information stream is modulated onto a radio carrier, and the modulated radio carrier is then transmitted over a radio link to a receiver of a base station. In the second communication path, the base station transmits a radio carrier modulated by digital information to the mobile phone, the modulated radio carrier is demodulated by a demodulator of the mobile phone, the demodulated waveform is passed to a digital-to-analog converter, and the analog output of the digital-to-analog converter is directed to a loudspeaker.
A mobile phone implementing the above communication paths comprises many discrete physical components packed into a small area. Consequently, electromagnetic energy of a particular frequency may escape from some of these components into the surrounding environment potentially causing noise interference to the other components of the mobile phone. Of particular concern to a designer of a mobile phone is the microphone and loudspeaker of the mobile phone, both of which are subject to picking up this noise interference from the other components of the mobile phone. This is because the wire connecting the microphone to the analog-to-digital converter and the wire connecting the digital-to-analog converter to the loud speaker are both potentially vulnerable to picking up any electromagnetic energy transmitted from any of the other components. A particular problem is the 217 Hz sending frequency radiated by a Time Division Multiple Access (TDMA) transmitter of a mobile phone operating in accordance with the Global System for Mobile Communications (GSM) standard. This noise interference when heard by human ears resembles the sound of a bumblebee and is thus known as bumblebee noise.
Previously, the problem of noise interference from other components has been solved by careful design of the wires to the loudspeaker and from the microphone. Common trial and error methods of design include trying different wire positions or using more expensive wires. However, this is not an efficient solution to the problem of electromagnetic interference because this solution requires an experimental arrangement of physical components by a skilled designer.
In view of the foregoing, it would be desirable to provide a technique for canceling noise interference (such as bumblebee noise) occurring in a loudspeaker communication path which overcomes the above-described inadequacies and shortcomings. More particularly, it would be desirable to provide a technique for canceling noise interference in a loudspeaker communication path in an efficient and cost effective manner.
According to the present invention, a technique is provided for canceling interference (e.g., bumblebee noise, noise, periodic interference) from a first audio signal leading to a loudspeaker. A measuring device measures energy of the loudspeaker to produce a measurement signal. An adaptive unit estimates from the measurement signal a plurality of coefficients. A plurality of multipliers multiply the plurality of coefficients by a plurality of periodic signals so as to produce a Fourier approximation of the interference. A summation unit combines the first audio signal having the interference and the Fourier approximation of the interference to produce a second audio signal having the interference suppressed.
In a further aspect of the present invention, the plurality of periodic signals include a plurality of sine waveforms and a plurality of cosine waveforms.
In still a further aspect of the present invention, a signal generator generates the plurality of sine waveforms and the plurality of cosine waveforms.
In yet another aspect of the present invention, the first audio signal includes a speech signal corrupted by the interference.
In another aspect of the present invention, the second audio signal is received by the loudspeaker.
In an aspect of the present invention, the energy is sound energy generated by the loudspeaker, and the measuring device is a microphone receiving a portion of the sound energy.
In another aspect of the present invention, the energy is electrical energy of the second audio signal, and the measuring device receives a portion of the electrical energy.
In one particular aspect of the present invention, the adaptive unit calculates the plurality of coefficients based on a least mean square of the interference and the Fourier approximation of the interference.
In another aspect of the present invention, a filter, coupled to the adaption unit, receives the measurement signal and filters from the measurement signal an error component comprising the interference and the Fourier approximation of the interference.
In yet another aspect of the present invention, the filter includes a phase lock loop centered at 217 kHz.
Another aspect of the present invention includes a transmitter, and the interference is a result of electromagnetic energy generated by the transmitter radiating the electromagnetic energy centered at a predetermined frequency. Typically, the predetermined frequency is approximately 217 Hz.